US Vice President Kamala Harris is set to begin her trip to Poland and Romania tomorrow as the Biden administration continues to show support for Ukraine and the US’ eastern NATO allies while Russia’s invasion of Ukraine enters a dangerous new phase.Harris’ trip will take place from Wednesday through March 11, and will include stops in Warsaw and Bucharest, according to a Friday announcement by the White House.She’ll meet with the leaders of both countries to coordinate on their response to Russia’s invasion and discuss how the US can further support Ukraine’s neighboring nations as they prepare to welcome Ukrainian refugees fleeing the conflict.The White House says the leaders will also “discuss their continuing support for the people of Ukraine through security, economic, and humanitarian assistance and our determination to impose severe economic consequences on Russia and those complicit in Russia’s invasion.”The trip comes as the Pentagon on Tuesday dismissed Poland’s proposal to transfer its MiG-29 fighter jets to the United States for delivery to Ukraine.Pentagon press secretary John Kirby said in a statement that the Pentagon did not believe Poland’s proposal was “tenable,” just hours after Polish officials released a statement saying the government was ready to deploy all of its MiG-29 fighter jets to US Air Force’s Rammstein Air Base in Germany so they could then be provided to Ukraine in its fight against Russia.Kirby said that the decision about transferring Polish-owned planes to Ukraine was “ultimately one for the Polish government,” adding that the proposal shows the complexities that the issue presents as Russia has made threatening statements over arms being provided to Ukrainians for use against Russian forces.The idea as laid out by Poland was too risky, Kirby said, as the US and NATO seek to avoid an outright conflict between the alliance and Russia.More background: The Defense Department statement was released Tuesday evening after the Polish proposal caught the Biden administration completely off guard, multiple sources told CNN.While Ukrainian President Volodymyr Zelensky has pleaded for more aircraft amid the Russian invasion, the offer had not been discussed with the US before making it public and Polish officials did not bring it up with Secretary of State Antony Blinken when he was recently in Poland either.US officials have privately weighed sending aircraft to Ukraine but have repeatedly noted the difficult logistical challenges of doing so.Poland’s surprise announcement complicates what had already been a high-stakes visit by Vice President Kamala Harris, who is due to land in Warsaw late Wednesday.Harris had been expected to discuss the fighter jet issue while in Poland, according to officials. The White House had previously said it was in discussions with the Polish government about a plan for Poland to supply Ukraine with its Soviet-era fighter jets and the US to backfill the planes with F-16s.Harris is still scheduled to depart Wednesday morning for Poland, but now there are intensive conversations within the administration about how to work with Poland to come to some sort of agreement that allows the jets to reach Ukraine.CNN’s Jeremy Herb, Kylie Atwood, Jennifer Hansler and Oren Liebermann contributed reporting to this post.